The Strangest Exoplanet Ever is Made of Hot Ice

Over the last decade or so, astronomers have discovered gobs of exoplanets—now numbering in the high quadruple digits. It often appears as if each new discovery is a little bit stranger than the last (take TrEs-2b: a planet made of a substance that’s darker than coal). None I can think of come close to topping Gliese 436 b (otherwise known as GJ 436 b)—an exoplanet located approximately 30 light-years from Earth toward the constellation of Leo—in terms of unusual properties.

Basic Properties:

The Neptune-sized planet orbits a star that is cooler, smaller and less luminous than the Sun: a red dwarf known as Gliese 436. The planet, in turn, completes one full orbit around its parent star in just 2 days and 15.5 hours. This short orbital period indicates that the planet in question is located very close to its star, perhaps orbiting Gliese 436 from one thirteenth of the distance between Mercury—the innermost planet in our solar system—and the Sun.

Because of this close proximity, the planet’s temperatures exceed 800 Kelvin (or 980° F), which is hot enough to permanently ward off water in liquid form. Our current models say that a planet like this—that is, one that is composed mostly of hydrogen gas, but with such high surface temperatures—should have significant quantities of methane in its atmosphere. Here is where Gliese 436 b presents us our first conundrum: The planet has more than 7,000 times less methane than it should. Yet, it does have a surprising abundance of carbon monoxide molecules.

And therein lies the second mystery… carbon monoxide should not be present to this degree, as it becomes scarce when temperatures soar above a certain threshold.

According to a scientist from the Kepler team, “Carbon, when it is cold, likes to hold onto hydrogen, but if it is hotter it likes to throw off the the hydrogen and steal oxygen from, say, water molecules, to make carbon monoxide.”

The cause of this perplexing discovery is still unknown, but whatever the case may be, the mystery of the missing methane still has astronomers scratching their heads.

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The Strangest Thing of All:

Moving onward, as I mentioned before, the planet is comparable to Neptune in size, but it is too compact to have a complete hydrogen makeup (like typical gas giants), yet is not compact enough to be considered a “super-Earth” exoplanet. Therefore, astronomers believe the planet may host a large concentration of an exotic form of water-ice—now known as “Ice-x”—on top of a rocky core. It may seem extraordinary, but this is no familiar substance; It can remain solid despite blisteringly hot temperatures—we’re talking so hot, it could totally melt your face off if you somehow managed to catch a drop of it in your mouth.

So, you may wonder how water can remain solid at temperature exceeding 400°C/700°F. That’s an excellent question. As it stands, it may be that the immense gravity of the planet is strong enough to compress the trace amounts of water vapor in its atmosphere, keeping it from reverting back to its natural form.

A peek inside Gliese 436 b (Image Credit: Dr. Jason Wright)

New Insights:

In the years since Gliese 436 b was discovered, many new developments have come to light.

In a press release from the University of Warwick, a team of researchers revealed that this planet is caving in to the pressures exerted on it by its parent star.

To understand what this means, we must go back to 2013, when spectral analysis of Gliese 436 b proved inconsistent with other observations, suggesting it may have clouds in its atmosphere.

At the time, Dr Heather Knutson (the lead author) remarked: “Either this planet has a high cloud layer obscuring the view, or it has a cloud-free atmosphere that is deficient in hydrogen, which would make it very unlike Neptune. Instead of hydrogen, it could have relatively large amounts of heavier molecules such as water vapor, carbon monoxide, and carbon dioxide, which would compress the atmosphere and make it hard for us to detect any chemical signatures.”

By studying its transmission spectrum once more, the researchers from Warwick learned that a large cloud is situated between us and GJ 436b. This cloud, which presumably escaped from its planet’s atmosphere, is largely composed of hydrogen, and it continues to trail behind.